The (basal) level of FSH measured during early menses is emerging as a predictor of ovarian competence. In this study, correlates of basal FSH were examined in 222 premenopausal women who were not using oral contraceptives and selected from either the general population or a clinic for women with family histories of ovarian cancer. Using analysis of variance, the effect on FSH by age, smoking history, and reproductive variables was examined. Dietary galactose (as a potential oocyte toxin) was estimated, and red cell activity of galactose-1-phosphate uridyl transferase (GALT) was measured. Qualitative features of GALT were described by its electrophoretic or molecular genetic patterns. Possession of GALT polymorphisms previously linked with low GALT activity, including the Q188R mutation of classic galactosemia or N314D mutation of the Duarte galactosemia variant, was associated with significantly higher FSH, even in the heterozygous state. Other factors significantly influencing FSH included age, smoking history, cycle length, and cycle regularity. No effect of current galactose consumption was found, and GALT activity was only weakly correlated (inversely) with FSH. Applying multiple linear regression, variables independently predictive of high FSH were age of 40 yr or more, current smoking, and possession of a GALT polymorphism.